You know, "Sunday," "Monday," "Tuesday," etc.?
Have you ever noticed that some of them are named after the Sun, Moon, and planets?
Obviously, "Sunday" is named after the Sun...
But has it ever occurred to you that "Monday" is the "Moon's day?"
"Saturday" is "Saturn's day..."
The other days are a bit more complicated.
In Latin, they used to all be named for planets and in other languages such as French, Italian, and Romanian they still are.
I took French for six years in school (though I don't remember it as well
as Pig Latin, or rather "Ig-pay Atin-lay") and can attest that Tuesday is named "Mardi" for Mars.
Wednesday is "Mercredi" named after Mercury.
Thursday is "Jeudi" for Jupiter.
And Friday is "Vendredi" for Venus.
In English, the name "Tuesday" originated in Old English and was "Tyr's day." Tyr was a god of combat and heroic glory in Norse mythology, whose Roman equivalent is the god Mars.
The name "Wednesday" is also from Old English, "Wodnesdaeg" for the Germanic god "Wodan," also known as "Odin," the highest god in Norse mythology.
The name "Thursday" is (surprise, surprise) Old English for "Thor's day" - for the Norse god of Thunder. And yet again, there's a connection to the Roman chief god, who wielded a mighty thunderbolt - Jupiter.
Friday is Old English for Frigedaeg, the day of Frige, the Germanic goddess of beauty. And, of course, the Roman goddess of beauty is Venus.
As for why there are seven days, it seems there are roughly four phases of the Moon in a month, each divided by seven days.
Even the ancient Hindus of India used a seven-day week, mentioning it in one of their sacred bibles, "The Ramayana" in 500 B.C., naming the days after (guess what?) the planets - bhaanu vaasara (Sun), indu vasaara (Moon), mangal vaasara (Mars), etc.
This practice of having seven-day weeks has been practiced the world over for millennia, (though prior to modern times, some places used an eight-day week, and different cultures still disagree as to which days start or end the week, and which is the "Sabbath").